1. Field of the Invention
This invention relates to micellar whey protein comprising casein micelle-like molecular aggregates formed by association. More particularly, this invention relates to micellar whey protein characterized in that it has excellent thermal stability and high solubility and a solution thereof forms a coagulum in the acid pH range without application of heat, a solution containing it, and a powder and coagulum obtained therefrom. They can be widely utilized as food materials. This invention also relates to foods utilizing the protein.
2. Description of the Prior Art
Whey protein is generally defined as milk protein exhibiting water solubility under acidic conditions, and is obtained as a by-product chiefly in the process of manufacturing cheese and casein. In spite of its high nutritive value, whey protein has not been fully utilized in the food industry and most of it is discarded by reason of the fact that it requires heating for purpose of gelation as described later (on the contrary, undesired gelation occurs as a result of heat sterilization or the like). Accordingly, it is very important from the viewpoint of environmental protection and effective utilization of resources to promote the use of whey protein as a food material. Generally, whey protein is denatured by heating to 60.degree. C. or above and undergoes gelation easily under certain conditions, for example, when heated at a pH of 6 or less Mulvihill, D. M. and Kinsella, J. E. (1987) Food Technol., 41(9), 102-111! and when heated in the presence of a salt Schmidt, R. H., Illingworth, B. L., Deng, J. C. and Cornell, J. A. (1979) J. Agric. Food Chem., 27(3), 529!. Such gelation is believed to take place because the higher-order structure of the protein changes in consequence of denaturation by heating, and the resulting increase of hydrophobic regions and of the reactivity of disulfide bonds causes the protein molecules to form a three-dimensional network structure.
Since whey protein has high nutritive value, it is used as a material for highly nutritious foods such as powdered milk for infants. Moreover, since whey protein has various functional properties (e.g., binding and emulsifying properties) in addition to the above-described heat-gelling properties, it is also utilized as a meat binder, fat substitute and the like Morr, C. V. (1989) in Developments in Dairy Chemistry, P. F. Fox (ed.), Vol. 4, p. 323, Elsevier Applied Sci., New York!. Thus, the utilization of whey protein for various purposes has been attempted. However, whey protein requires heat treatment for purposes of gelation and has to be heated under static conditions. Consequently, it is not practical to manufacture a large amount of gel on an industrial scale. Moreover, a long time is required to cause gelation of the central part as a result of heat transfer from the periphery, which may result in scorching or pore formation due to overheating. Furthermore, the utilization of whey protein is also limited by the fact that it cannot be used in non-heat-processed foods. When whey protein is used in drinks and other foods in which gelation should be inhibited, the whey protein may be aggregated as a result of heat sterilization or the like. This makes it difficult to apply whey protein to drinks and the like. Owing to these problems, whey protein is still used only as a supplemental material in a limited field of food processing.
Accordingly, there has been developed a whey-protein-containing solution which does not require heat treatment for purposes of gelation but undergoes gelation by the action of monovalent or divalent salt-derived ions (U.S. Pat. No. 5,217,741). This solution is obtained by heating a whey protein solution having a low salt-derived ion concentration as defined by an ash content of not greater than 1% to increase the degree of hydrophobicity of the whey protein. The salt-induced gel obtained by the addition of salt-derived ions to the solution is a smooth and transparent gel having excellent water-holding properties and enables whey protein to be applied to non-heat-processed foods. However, since this gel is induced by the addition of salt-derived ions, a taste arising from the salt is left. Moreover, the powder obtained by drying the heated whey protein solution shows a marked reduction in processability because its solubility is very low.